Off-road electric vehicles, such as lift trucks, sweepers and scrubbers and ground support equipment, are used in a variety of commercial and recreational applications. By way of example, electric lift trucks comprising pallet forks are commonly used in retailing, wholesaling and manufacturing operations for lifting and moving materials inside warehouses and the like. Since lift trucks are often operated indoors, the use of internal combustion engines is precluded. In most cases lift trucks are battery powered to avoid potentially harmful emissions. Each battery is mounted within an enclosure comprising a battery receptacle tray or cavity typically located near the rear of the vehicle (although the location varies depending upon the vehicle model). The batteries typically include handles or lifting grips and the receptacle tray may include rollers to facilitate battery movement, for example during recharging operations. When in use, the battery output is electrically connected to the vehicle drive system with a DC interface plug.
Various types of lead acid battery systems are available for use in lift trucks and other similar electric vehicles. Flooded battery systems provide approximately 6–8 hours of operation and require frequent watering to maintain the chemistries in their cells as they are charged and discharged. Batteries requiring less frequent watering, such as “Water-less”™battery systems manufactured by Hawker Powersource, are also available and provide similar performance to flooded batteries. Recently “maintenance free” battery systems have been introduced which do not require any watering, but require more expensive chargers. Maintenance-free systems have a lower energy storage capacity per cubic foot and therefore provide fewer hours of operation than flooded or reduced water batteries of the same size.
All conventional battery systems designed for low power vehicular applications suffer from serious shortcomings. A primary limitation is that conventional batteries must be recharged at frequent intervals, usually at least every 6–8 hours. Accordingly, battery charging stations must be provided at the worksite. The establishment of a battery charging infrastructure is costly and occupies valuable warehouse space. Moreover, the vehicles cannot be continuously operated (i.e. in sequential shifts) without routinely swapping discharged and charged batteries. This frequent daily removal of discharged batteries and substitution of fully charged batteries is labour-intensive and potentially dangerous (conventional battery enclosure systems for Class A lift trucks weigh up to 900 pounds). In order to be effective, such battery swapping also requires multiple batteries per vehicle which increases operating costs.
Conventional batteries must also be serviced at frequent intervals for cleaning and watering. The presence of battery acid poses employee safety risks and the potential to damage equipment.
Further, conventional battery systems are incapable of operating at optimum efficiency in many industrial applications. As shown in FIG. 14, lift trucks typically have a pattern of power usage or “duty cycle” which is characterized by loads which fluctuate substantially during the course of a work shift. For example, although the average load across an entire seven hour work shift is less than 1 kW, power requirements on the order of 8–10 kW for short durations are required at irregular intervals to meet operational demands. The state of charge of the battery must always be high enough to ensure that the battery is capable of responding to high current requests by the lift truck (even though the average power requirement is relatively low). This decreases the effective charge life of the battery, requiring recharging at more frequent intervals and resulting in operating downtimes.
The use of fuel cell power systems in industrial vehicles as an alternative to battery power is well known in the prior art. Fuel cell systems offer many important benefits including extended operating times, low emissions and the flexibility to utilize readily available fuels, such as methanol and propane (LPG). Further, the need for a battery charging infrastructure as described above is avoided, including the need for multiple batteries.
Notwithstanding these advantages, previous attempts by original equipment manufacturers (OEMs) to integrate fuel cell power systems employing conventional fuels into industrial trucks at a reasonable cost have been largely unsuccessful. It is not feasible to adapt existing trucks to fuel cell power without making extensive truck-level modifications. Each OEM brand truck requires a unique integration approach which is often difficult and expensive to implement, especially for existing fleets of vehicles. Moreover, if the fuel cell system fails, the truck must be taken out of service.
The fact that duty cycles for lift trucks and other similar vehicles are characterized by very high peak to average load ratios poses particular operational challenges. Many fuel cell systems employ reformers which convert conventional fuels into hydrogen-enriched gas which the fuel cell system transforms into electricity. However, this reforming process is relatively slow which limits the load following capabilities of the fuel cell. Also, in order to maximize the useful life of fuel cell components, it is preferable to operate the fuel cell at near steady state conditions rather than adopting a load following approach.
Some hybrid power supply systems are known in the prior art for use in applications subject to sudden load fluctuations. U.S. Pat. No. 4,883,724, Yamamoto, issued Nov. 28, 1989 relates to a control unit for a fuel cell generating system which varies the output of the fuel cell depending upon the state of charge of the battery. In particular, a DC/DC converter is connected between the output of the fuel cell and the battery and is responsive to a control signal produced by a controller. The purpose of the Yamamoto invention is to ensure the storage battery is charged for recovery within the shortest possible time to reach a target remaining charge capacity under charging conditions that do not cause deterioration of performance of the battery. When the charged quantity of the battery is recovered to the target value, the controller lowers the output of the fuel cell to its normal operating state. In the case of no external load, such as during extended periods of interruption in the operation of the lift truck, the fuel cell is controlled to stop after the storage battery is charged.
The primary limitation of the Yamamoto control system is that control algorithm is designed for prolonging the useful life of the storage battery rather than the fuel cell. By varying the fuel cell output to charge the storage battery for recovery within the shortest possible time, the long-term performance of the fuel cell is compromised. Moreover, Yamamoto does not disclose a hybrid fuel cell system which is configured to fit within a small geometric space.
The need has accordingly arisen for a hybrid architecture specifically adapted for lift trucks and other low power applications which integrates fuel cell technology with conventional battery systems. In the present invention the fuel cell and fuel processor systems are sized to meet the average load requirements of the vehicle, while the batteries and power control hardware are capable of responding to very high instantaneous load demands. The invention may be substituted for conventional batteries to improve performance without retrofitting existing fleets of vehicles. As described further below, the applicant's invention fits into conventional lift truck battery receptacle trays and has a similar electrical interface as conventional battery systems. Apart from vehicular applications, low power hybrid fuel cell products as exemplified by the present invention may also find application in uninterruptable power supply systems, recreational power, off-grid power generation and other analogous applications.